Electrolyser System

ABSTRACT

An electrolyser system comprises a water electrolyser having first and second electrode compartments, and a vessel having first and second chambers, the first compartment connected to the first chamber and the second compartment connected to the second chamber, via valved ports, wherein the first chamber also comprises a valved outlet and the second chamber also comprises a valved inlet, and wherein the system includes pressure sensing means to open or close the valves.

CROSS-REFERENCE TO A RELATED APPLICATION

This application claims priority to Great Britain Application No.0714140.1, filed Jul. 19, 2007, which is incorporated herein in itsentirety.

FIELD OF THE INVENTION

This invention relates to an electrolyser system for militating againstosmotic drag.

BACKGROUND OF THE INVENTION

An ionic exchange membrane electrolyser consists of two catalyticelectrodes separated by an ionically conductive solid polymerelectrolyte. Cationic and Anionic material may be used to form the solidpolymer electrolyser membrane. In both cases if the electrolyseroperates in a system which is not open to the surrounding environment(closed system) it will pressurise at a rate dependent upon the volumeof the head space within the separation towers (the vessels that eithersupply water to, or collect gas and water from, the electrolyser),system temperature and the gas production rate.

Electro-osmotic drag occurs in all solid polymer electrolyte membraneelectrochemical cells. It is a process by which water is transportedthrough the membrane in the direction of ion transport (i.e. forcationic exchange (CE) systems, from the oxygen to the hydrogen side).The degree of drag is directly related to the operational current of theelectrolyser, the temperature of the system and the chemistry of themembrane. Therefore, for CE closed electrolyser systems this will causethe oxygen side of the electrolyser to become devoid of water and topressurise more slowly than predicted, simultaneously the hydrogen sidewill have an increased volume of water and pressurise at a greater thanpredicted rate.

In order to combat the effects of osmotic drag the water needs to beremoved from the hydrogen separation vessel and water needs to be addedinto the oxygen separation vessel and the differential pressure betweenthe separation towers needs to be equalised in order to reduce thedifferential pressure across the membrane. Due to the risk of explosion,it is not possible to transfer water from the hydrogen tower to oxygentower directly, since dissolved hydrogen gas within the transferredwater may come out of solution, creating an explosive mixture ofhydrogen and oxygen gas within the restricted space of the pressurisedoxygen separation vessels; therefore fresh water needs to be used.

The reverse is true for an anionic exchange (AE) system; the drag willcarry water from the hydrogen side to the oxygen side, resulting in adecreased rate of pressurisation in the hydrogen vessel and an increasedrate of pressurisation in the oxygen vessel. This can be combated byremoving water from the oxygen vessel and introducing fresh water to thehydrogen vessel.

SUMMARY OF THE INVENTION

The present invention is based on the realisation that it is possible toutilise the different pressures within a closed system of anelectrolyser to drive a “pump”. This “pump” can inject fresh water froma low pressure source into the highly pressurised vessel minimising thedifferential pressure across the electrolyser membrane and resulting inimproved water management.

According to a first aspect, the present invention is an electrolysersystem comprising a water electrolyser having first and second electrodecompartments, and a vessel having first and second chambers, the firstcompartment connected to the first chamber and the second compartmentconnect to the second chamber, via valved ports, wherein the firstchamber also comprises a valved outlet and the second chamber alsocomprises a valved inlet, and wherein the system includes pressuresensing means to open or close the valves.

According to a second aspect, the present invention is a method formilitating against osmotic drag in an electrolyser having first andsecond electrode compartments and that produces hydrogen and oxygen byelectrolysis of water, which comprises removing water from the firstelectrode compartment into the first chamber of a vessel, and supplyingwater from the second chamber of the vessel into the second electrodecompartment, wherein the flow of water is controlled by the relativepressures in the first and second electrode compartments.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 illustrates an electrolyser system of the invention and alsoillustrates a vessel for operation with a CE system, in which the Ph>Po;the reverse is true for AE systems.

DESCRIPTION OF PREFERRED EMBODIMENTS

The electrolyser system comprises a water electrolyser (which produceshydrogen and oxygen) having first and second electrode compartments anda vessel having first and second chambers. The first chamber isconnected to the first electrode compartment via a valved port, and thesecond chamber is connected the second electrode compartment via avalved port. In one embodiment, the first electrode compartment is thecathode and the second electrode compartment is the anode. In anotherembodiment, the first electrode compartment is the anode and the secondelectrode compartment is the cathode.

The, or each, valved port may comprise a separation tower. A “separationtower” or “separation vessel”, as described herein, is a containersuitable for positioning between the water electrolyser and the vessel,in an electrolyser system of the invention. It may be connected to thevessel and the electrolyser, via valved ports. Alternatively, it may beintegral with the electrolyser.

When the first electrode compartment is the cathode and the secondelectrode compartment is the anode, the tower separating the firstcompartment and the first chamber will contain H₂/water mix, and thetower separating the second compartment and the second chamber willcontain an O₂/water mix. When the first electrode compartment is theanode and the second electrode compartment is the cathode, the towerseparating the first electrode compartment and the first chamber willcontain an O₂/water mix, and the tower separating the second electrodecompartment and the second chamber will contain a H₂/water mix.

The separation tower may be a pespex cylindrical tower. It may have avalved port near the bottom of the tower to supply water to theelectrolyser, and a valved port near the top of the tower to acceptwater/gas from the electrolyser. The separation tower allows the watersto be re-circulated and degassed after passing through the electrolyser.As the gas builds up, it may be transferred to a storage container.

The pressure sensing means may sense pressure in the first and secondchambers of the vessels and/or in the first and second electrodecompartments. The pressure sensing means may either directly orindirectly cause the opening and closing of the valves.

Preferably the pressure sensing means is a moveable member separatingthe first and the second chambers of the vessel, so that the contents ofthe two chambers are isolated from each other. The moveable memberpreferably moves between a first and second position. When it reachesthe first position, it causes the valves of the ports to open and thevalves of the inlet and the outlet to close. When it reaches the secondposition, it causes the valves of the ports to close and the valves ofthe inlet and outlet to open.

In one embodiment, the member opens or closes the valves by activating aswitch at either the first or second position. The switches may bemicro-switches, which are placed either inside the chambers, orexternally. For external switching the moveable member may have an armwhich projects out of the chamber, which can then activate externalmicro-switches.

Mechanical actuators may be used to open and close the valves. These aredriven/commanded by micro-switches, which can either be intelligentlycoupled to pressure sensors within the system, or connected to anexternal arm, which is connected to the piston. As the piston moves, sotoo does the arm, which makes/breaks electrical connections, andcommands the valves to open or close.

The invention will now be described, by way of example only, withreference to the accompanying drawing.

FIG. 1 illustrates an electrolyser system of the invention. It shows avessel suitable for operation with a cationic exchange (CE) electrolyser(not shown).

The vessel (FIG. 1) comprises:

-   -   four valves: Vh is connected to a hydrogen separation vessel        (not shown) of an electrolyser; Vo is connected to an oxygen        separation vessel (not shown) of an electrolyser; Vd is        connected to a drain; and Vf is connected to a fresh water        supply.    -   a sliding section defining Chamber A and Chamber B, within a        high pressure cylinder capable of moving between positions A and        B, and containing four ports (each connected to an appropriate        valve).    -   switches at positions A and B to activate the opening and        closing of the valves.

At the start of the electrolyser operation, Vf and Vd are closed, and Vhand Vo are open. The pressure in chamber section A is therefore equal tothe pressure in the hydrogen separation vessel (Ph) and the pressure inchamber section B is equal to the pressure in the oxygen separationvessel (Po). Before operation, Ph is equal to Po; during operation, Pfis higher than Pd.

During operation the electrolyser will produce hydrogen and oxygen in a2:1 ratio, if the gas towers are of equal size the system willpressurise so that the pressure in the hydrogen vessel (Ph) becomesgreater (approximately twice) than the pressure in the oxygen vessel(Po). The water levels will change so that the water level in thehydrogen vessel (Wh) increases (due to osmotic drag influx) and thewater level in the oxygen vessel (Wo) will decrease (due to losses fromosmotic drag and the utilisation of water for electrolysis), furtherincreasing the pressure differential between Ph and Po. Since valves Vhand Vo are open the pressures in section A and section B will correspondto the pressures in the respective vessels.

As Ph increases at a faster rate than Po, the sliding section in thechamber will be pushed from position A to position B in an attempt toequalise the pressure in each of the chambers. Once the sliding sectionreaches position B, a micro-switch closes valves Vo and Vh, and opens Vdand Vf. When Vd opens to the atmosphere, the hydrogen water will drainout of the chamber, as the pressure in chamber A is greater thanatmospheric pressure; the pressure in section A will therefore decrease.Fresh water will be pushed into section B through the open valve, fromthe water source (which has a pressure above atmospheric), causing thesliding mechanism to move from position B to position A. A mains watersupply is at adequate pressure to cause the sliding mechanism to movefrom B to A. This reduces the need for a high pressure water pump. Thismeans both reduced capital, operational and maintenance costs. Once atposition A, a micro-switch will close valves Vd and Vf and open valvesVh and Vo, again allowing the sliding mechanism to move towards positionB to negate the pressure build-up. This can be used as a continuoussystem.

FIG. 1 illustrates a vessel for operation with a CE system, in which thePh>Po; the reverse is true for AE systems. AE systems require water tobe removed from the O₂ vessel and introduced into the H₂ vessel; thisrequires Po>Ph. Simple modifications to the vessel design will allow thecassette to operate with an AE system. Specifically, the H₂ vessel musthave a volume of more than twice the volume of the O₂ vessel.

1. An electrolyser system comprising a water electrolyser having firstand second electrode compartments, and a vessel having first and secondchambers, the first compartment connected to the first chamber and thesecond compartment connected to the second chamber, via valved ports,wherein the first chamber also comprises a valved outlet and the secondchamber also comprises a valved inlet, and wherein the system includespressure sensing means to open or close the valves.
 2. The systemaccording to claim 1, wherein the, or each, valved port comprises aseparation tower.
 3. The system according to claim 1, wherein thepressure sensing means comprises a member separating the first andsecond chambers, moveable between first and second positions, wherebythe member causes the ports to open and the inlet and outlet to closewhen it reaches the first position, and causes the ports to close andthe inlet and outlet to open when it reaches the second position.
 4. Thesystem according to claim 3, wherein the member causes the opening orclosing of the ports, the inlet and the outlet, by activating switchesat the first and second positions.
 5. The system according to claim 1,wherein the inlet is connected to a water supply.
 6. The systemaccording to claim 1, wherein the first chamber is connected to thecathode compartment of the electrolyser, and the second chamber isconnected to the anode compartment.
 7. The system according to claim 1,wherein the first chamber is connected to the anode compartment of theelectrolyser, and the second chamber is connected to the cathodecompartment.
 8. A method for militating against osmotic drag in anelectrolyser having first and second electrode compartments and thatproduces hydrogen and oxygen by electrolysis of water, which comprisesremoving water from the first electrode compartment into the firstchamber of a vessel, and supplying water from the second chamber of thevessel into the second electrode compartment, wherein the flow of wateris controlled by the relative pressures in the first and secondelectrode compartments.
 9. The method according to claim 8, wherein thefirst electrode compartment is the cathode and the second electrodecompartment is the anode.
 10. The method according to claim 8, whereinthe first electrode compartment is the anode and the second electrodecompartment is the cathode.